Many gas streams contain components which are undesirable and which need to be removed from the gas stream prior to its discharge to the atmosphere or further processing. One such component is hydrogen sulfide, while another such component is sulfur dioxide.
Hydrogen sulfide occurs in varying quantities in many gas streams, for example, in sour natural gas streams and in tail gas streams from various industrial operations. Hydrogen sulfide is odiferous, highly toxic and a catalyst poison for many reactions and hence it is desirable and often necessary to remove hydrogen sulfide from such gas streams.
There exist several commercial processes for effecting hydrogen sulfide removal. These include processes, such as absorption in solvents, in which the hydrogen sulfide first is removed as such and then converted into elemental sulfur in a second distinct step, such as in a Claus plant. Such commercial processes also include liquid phase oxidation processes, such as Stretford, LO-CAT, Unisulf, Sulferox, Hiperion and others, whereby the hydrogen sulfide removal and conversion to elemental sulfur normally are effected in reaction and regeneration steps.
In Canadian Patent No. 1,212,819 and its corresponding U.S. Pat. No. 4,919,914, the disclosure of which is incorporated herein by reference there is described a process for the removal of hydrogen sulfide from gas streams by oxidation of the hydrogen sulfide at a submerged location in an agitated flotation cell in intimate contact with an iron chelate solution and flotation of sulfur particles produced in the oxidation from the iron chelate solution by hydrogen sulfide-depleted gas bubbles.
The combustion of sulfur-containing carbonaceous fuels, such as fuel oil, fuel gas, petroleum coke and coal, as well as other processes, produces an effluent gas stream containing sulfur dioxide. The discharge of such sulfur dioxide-containing gas streams to the atmosphere has lead to the incidence of the phenomenon of "acid rain" which is harmful to a variety of vegetation and other life forms. Various proposals have been made to decrease such emissions.
A search in the facilities of the United States Patent and Trademark Office with respect to gas-liquid contacting procedures has revealed the following United States patents as the most relevant to the present invention:
______________________________________ U.S. Pat. No. 2,274,658 U.S. Pat. No. 2,294,827 U.S. Pat. No. 3,273,865 U.S. Pat. No. 4,683,062 U.S. Pat. No. 4,789,469 ______________________________________
U.S. Pat. Nos. 2,274,658 and 2,294,827 (Booth) describe the use of an impeller to draw gas into a liquid medium and to disperse the gas as bubbles in the liquid medium for the purpose of removing dissolved gaseous materials and suspended impurities from the liquid medium, particularly a waste stream from rayon spinning, by the agitation and aeration caused by distribution of the gas bubbles by the impeller.
The suspended solids are removed from the liquid phase by froth flotation while the dissolved gases are stripped out of the liquid phase. The process described in this prior art is concerned with contacting liquid media in a vessel for the purpose of removing components from the liquid phase.
These references contain no discussion or suggestion for removal of components from gas streams by introduction to a liquid phase. In addition, the references do not describe any critical combination of impeller--shroud parameters for effecting such removal, as required herein.
U.S. Pat. No. 3,273,865 describes an aerator for sewage treatment. A high speed impeller in the form of a stack of flat discs forms a vortex in the liquid to draw air into the aqueous phase and circulate the aqueous phase. As in the case of the two Booth references, this prior art is concerned solely with aeration of a liquid phase to treat liquid phase components. In addition, the reference does not describe or suggest an impeller-shroud combination for effecting such removal, as required herein.
U.S. Pat. No. 4,683,062 describes a perforated rotatable body structure which enables liquid/solid contact to occur to effect biocatalytical reactions. This reference does not describe an arrangement in which gas-liquid contact is effected.
U.S. Pat. No. 4,789,469 describes the employment of a series of rotating plates to introduce gases to or remove gases from liquids. There is no description or suggestion of an impeller-shroud combination, as required herein.
Many other gas-liquid contactors and flotation devices are described in the literature, for example:
(a) "Development of Self-Inducing Dispenser for Gas/Liquid and Liquid/Liquid Systems" by Koen et al, Proceeding of the Second European Conference on Mixing, 30th March-1st April 1977;
(b) Chapter entitled "Outokumpu Flotation Machines" by K. Fallenius, in Chapter 29 of "Flotation" ed M. C. Fuerstenau, AIMM, PE Inc, New York 1976; and
(c) Chapter entitled "Flotation Machines and Equipment" in "Flotation Agents and Processes, Chemical Technology Review #172" M. M. Ranhey, Editor, 1980.
However, none of this prior art describes the impeller-shroud structure used herein.